In a press conference Monday, Trump said: “One way or another, Mexico is going to pay for that wall.”

The wall and who is paying for it has seeped into a bevy of political issues. Last week, he threatened a government shutdown over the wall’s funding. (Trump expects the U.S. to be reimbursed by Mexico.) This would affect the Trump administration’s agenda, and continue to distract and wreak havoc on tax reform and hopes for bipartisanship.

But if Congress were to approve funds for a southern border wall, they should not expect to be reimbursed. It is impossible to overstate exactly how much Mexico does not want to pay for this wall, no matter how many times Trump insists otherwise.

The U.S. public’s support for the wall is divided at best, but in Mexico opposition to paying for it is completely united. REUTERS/Ashlee Espinal

Agreeing to pay for the wall would be “treason to the motherland”

In response to Trump’s mid-hurricane tweet, the Mexican foreign ministry issued a statement, reiterating it would not pay “under any circumstances,” and adding an important note that is key to understanding the Mexican-American standoff: “This statement is not part of a Mexican negotiating strategy, but rather a principle of national sovereignty and dignity.”

Trump continues to view the opposition from Mexico as a negotiating tactic.

“You cannot say that to the press because I cannot negotiate under those circumstances,” Trump responded to Mexico president Enrique Peña Nieto’s rejection during a call.

“Most Mexicans see Trump’s threat as an attack on the country’s honor,” said Fernando Dworak, a political analyst in Mexico City. “Any politician who would suggest any collaboration with the USA on building the wall would immediately be seen as a traitor and a nation-seller (vende patrias).”

To Mexico, the demand to pay for a wall would be as large as a demand to change the name of the country or the flag.

“There’s no other issue of Trump’s long-standing anti-Mexico tirade that grates Mexican public opinion more than stating that Mexico will pay for the wall,” Arturo Sarukhan, Ambassador of Mexico to the U.S. from 2006 to 2013, told Yahoo Finance.

The wall has united Mexico against the U.S.

“If Trump expected a mild response from Peña Nieto, as leaks have suggested, he made a big tactical mistake,” said Dworak. “Mexico has developed a defensive nationalism since the 19th century.”

Trump’s comments have brought Mexicans together and strengthened their nationalism. Since Trump’s “drugs, crime, rapists” speech, Mexico’s public opinion of the U.S. has fallen considerably, with 49% of Mexicans polled in January having a “bad” or “very bad” view of the U.S. Meanwhile, a February Gallup poll found the U.S.’s favorable view of Mexico at its highest in a decade.

“It’s no wonder that pimping Mexico as a political and electoral piñata with his base has triggered a collapse of favorable opinions of the U.S. to an all-time low in recent Mexican history,” said Sarukhan. “The wall has become a third-rail for Mexico-U.S. ties.”

For Mexico, a wall is a slap in the face

In the leaked calls between Peña Nieto and Trump, Peña Nieto reiterated his view that the U.S. can do what it wants on its own land. “I have recognized the right of any government to protect its borders as it deems necessary and convenient,” he told Trump. “But Mexico cannot pay for that wall.”

Still, however, Mexicans find the idea disrespectful.

“The idea of a wall — it’s offensive,” said Alejandro Hope, a security analyst and columnist for El Universal, a major Mexico City newspaper. “The notion of actually paying for it is adding insult to injury. It’s simply not negotiable. It’s an issue of national pride and national dignity.”

According to Hope, many Mexicans contend that if anything, the money should flow the other way to compensate for U.S. transgressions. The Bureau of Alcohol, Tobacco, and Firearms found that 70% of seized guns in Mexico came from the U.S., contributing to the tens of thousands of murders from organized crime south of the border. Furthermore, in Mexico, there is resentment of thirsty American noses that keep drug demand high.

As the Mexican foreign ministry statement read: ”It is a shared problem that will only end if its root causes are addressed: high demand for drugs in the United States and supply from Mexico (and other countries).”

Compelling Mexico to pay without consent is seen as “extortion” or “theft”

There are scenarios like tariffs and taxes in which Trump could attempt to get reimbursed without Mexico’s consent, though there are considerable downsides.

“It’s impossible for Mexico to agree to [pay for the wall],” Hope said. “Trump might extort payment, but it’ll be seen in Mexico as extortion or theft. Mexico could strike back in other ways.”

Even before that, however, things get tough for “reimbursement.” Options like tariffs or border adjustment taxes explicitly tied to the wall might be able to give Trump a political win for “getting Mexico to pay,” but they wouldn’t really guarantee that Mexico will foot the bill. In the end, those unpopular options would likely end up costing American consumers, as the higher costs for goods would simply be passed down the chain.

Perhaps the best way to understand the wall situation from the Mexican perspective is a massive game of chicken between a truck and a train. Mexico’s position is on rails and simply cannot turn.

“In the past two decades, Mexico and the U.S. have done — and can continue to do — a great many things together,” said Sarukhan. “The one thing Mexico won’t do jointly with the U.S. is build a wall.”

Ethan Wolff-Mann is a writer at Yahoo Finance focusing on consumer issues, tech, and personal finance. Follow him on Twitter@ewolffmann. Got a tip? Send to: tips@yahoo-inc.com.